Method for communicating in a MIMO network

ABSTRACT

The present invention relates to a method for communicating in a network, said network comprising a primary station and at least a first secondary station, wherein the first secondary station transmits to the primary station an indication of a first plurality of precoding vectors, wherein the number of first precoding vectors is greater than a preferred rank of transmission from the primary station to the first secondary station.

CLAIM OF PRIORITY

This application claims the benefit or priority of and describes therelationships between the following applications: wherein thisapplication is a continuation of U.S. patent application Ser. No.13/257,136, filed Sep. 16, 2012, which is a National Stage ofInternational Application No. PCT/IB10/51130 filed Mar. 16, 2010, whichclaims foreign priority from EP application EP09155424 filed Mar. 17,2009, all of which are incorporated herein in whole by reference.

FIELD OF THE INVENTION

The present invention relates to a method for communicating in acommunication network. More specifically, it relates to a method forcommunicating between a primary station and one or more secondarystations, in a MIMO (Multiple Input Multiple Output) mode. It alsorelates to primary stations or secondary stations able to implement sucha method.

This invention is, for example, relevant for all wireless communicationnetworks, and in an example of the following description, for a mobiletelecommunication network such as UMTS, or UMTS LTE.

BACKGROUND OF THE INVENTION

In communication networks, in order to increase the reachable throughputof communication, MIMO (Multiple Input, Multiple Output) has beenproposed widely. MIMO involves the use of multiple antennas at both thetransmitter and receiver to improve communication performance. It indeedoffers significant increases in data throughput without additionalbandwidth or transmit power by higher spectral efficiency (more bits persecond per hertz of bandwidth) and link reliability.

Multi User MIMO (MU-MIMO) is an advanced MIMO, allowing a station tocommunicate with multiple users in the same band simultaneously. In anexemplary embodiment of the invention, a mobile communication networkcomprises a primary station (base station, or NodeB or eNodeB) which cancommunicate simultaneously with a plurality of secondary stations(mobile stations, or User Equipment, or UE) with MIMO streams, by usinga plurality of primary station antennas and a plurality of secondarystation antennas. In order to form the stream, the secondary stationsprovide the primary station with information related to the state of thechannel by transmitting CSI (channel state information) feedback to theprimary station. Such CSI indicates an optimal or at least a preferredprecoding vector to be used in order to maximise the reachable data rateof the corresponding spatially separable data stream transmitted by theprimary station. This precoding vector can be a set of complex values tobe applied to each antenna port of the primary station duringtransmission to direct the data stream towards the secondary stationantennas.

However, in the context of MU-MIMO, the signaled precoding vector whenused may cause a beam interfering with another secondary stationcommunicating at the same time with the primary station. Moreover, thesecondary station is not able to evaluate where interfering stations areand whether the use of a precoding vector can cause interference. Thesharing of the precoding vectors transmitted by the secondary stationsin each secondary station would cause too much signaling and wouldrequire too much calculation power of each secondary station to generatenon interfering precoding vectors.

SUMMARY OF THE INVENTION

It is an object of the invention to propose an improved method forcommunicating in a MU-MIMO network which alleviates the above describedproblems.

It is another object of the invention to propose a method forcommunicating which does not cause too much signaling while enhancingthe channel quality by reducing interference.

Still another object of the invention is to propose a system comprisinga primary station and secondary stations that can maximise the datathroughput of the whole system.

To this end, in accordance with one aspect of the invention, a method isproposed for communicating in a network, said network comprising aprimary station and at least a first secondary station, wherein thefirst secondary station transmits to the primary station an indicationof a first plurality of precoding vectors, wherein the number of firstprecoding vectors is greater than a preferred rank of transmission fromthe primary station to the first secondary station.

What is meant by rank of transmission is the number of spatiallyseparable data streams of the MIMO communication between the primarystation and a given secondary station. It is to be noted that the rankcannot exceed the minimum of the number of antennas of the primarystation and of the secondary station. For instance, a secondary stationhaving four antennas cannot receive more than four spatially separablestreams, so cannot exceed rank-4 communications. Moreover, asixteen-antenna primary station cannot transmit more than 16 beams. Asan example, such a primary station could transmit simultaneously fourrank-4 MIMO transmissions to four secondary stations, or one rank-4 MIMOtransmission to one secondary station with two rank-2 MIMO transmissionsto another two secondary stations and eight rank-1 MIMO transmissions toanother eight secondary stations.

As a consequence, the primary station is able to generate anotherprecoding vector, for instance based on a linear combination of theplurality of vectors indicated in order to establish the communication.In the case of a MU-MIMO embodiment, the primary station is now able togenerate a precoding vector that may be suboptimal from the point ofview of the secondary station, but which permits to prevent interferencebetween streams transmitted to different secondary stations. In aparticular embodiment, the primary station selects a combination ofprecoding vectors, like a linear combination which does not require alot of processing, so that the sum rate of all the transmission rates ofthe connected secondary stations is maximal.

In accordance with another aspect of the invention, a secondary stationis proposed comprising means for communicating in a network with aprimary station, the secondary station further comprising transmittingmeans arranged for transmitting to the primary station an indication ofa first plurality of precoding vectors, wherein the number of firstprecoding vectors is greater than a preferred rank of transmission fromthe primary station to the first secondary station.

These and other aspects of the invention will be apparent from and willbe elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of a network in accordance with a beamformingscheme maximizing the rate of one secondary station;

FIG. 2 is a block diagram of a network in accordance with an embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a communication network having aprimary station and a plurality of secondary stations communicating withthe primary station. Such a network is illustrated for example in FIGS.1 and 2, where a primary station or base station 100 communicateswirelessly with a plurality of secondary stations 101, 102, 103 and 104.In an illustrative example of the invention, the secondary stations101-104 are mobile stations or user equipment of a UMTS network.

In accordance with a first embodiment of the invention, the primarystation 100 comprises an antenna array comprising a plurality ofantennas and a complex gain amplifier so that the primary station 100 isable to carry out beamforming, like MIMO beamforming Typically, theprimary station comprises four antennas. In the most advanced versionsof LTE, the primary stations may comprise 8, 16 antennas or more.Similarly, the secondary stations 101-104 comprise a plurality ofantennas, e.g. 2 antennas for the UEs compliant with the first LTErelease. In later releases, the secondary stations may have 4 or 8antennas, or even more. Thanks to the antenna arrays, the primarystation 100 can form beams of data streams, like the beams 150 and 151depicted on FIG. 1. In order to form the beam and establish a MIMOcommunication, the generation of precoding vectors is essential, thisgeneration requiring information about the state of the channel andcomputation on both the secondary station and the primary station sides.

For example, in the first release of the LTE specifications, thesecondary stations configured to receive downlink transmissions inMU-MIMO and to make measurements of the downlink channel (typicallyusing non-precoded common reference signals (CRS)) and transmit channelstate information (CSI) feedback to the primary station, the eNodeB.This indicates a preferred precoding vector to be used for the downlinktransmissions (PMI, precoding matrix indicator) and an associated CQI(Channel Quality Information) value indicating a correspondingmodulation and coding scheme. In this example, the downlinktransmissions are codebook based, meaning that the precoding vectorsused for transmission are selected from a finite set. The chosenprecoding vector is signaled to the secondary stations so that thesecondary station can derive a phase reference as a corresponding linearcombination of the Common Reference Signals (CRSs).

A secondary station with a single receive antenna feeds back the indexof a single preferred precoding vector which enables the best qualitytransmission or the most reliable communication, for example the onewhich maximises the signal to interference ratio SINR at its antenna.This can be based on a predetermined codebook of transmit beamformingvectors, or direct channel vector quantisation (CVQ). In case thesecondary station has two (or more) receive antennas, the situation ismore complex and the approach taken depends on the size of the codebookavailable for the quantised CSI feedback. What could be done at such asecondary station would be to feed back the full channel matrix (or atleast a quantised version of it). This would however require significantsignaling overhead and resource.

In case of Rank-2 transmission, it is possible to feed back a preferredprecoding matrix. This is however not appropriate if the secondarystation prefers Rank 1 transmission, for example due to the rank of thechannel matrix being limited, or if the secondary station is configuredin a MIMO mode which only supports Rank-1 transmission, or if theprimary station schedules only a Rank-1 transmission.

For Rank-1 transmission, in the case of a relatively small feedbackcodebook, it is feasible for a secondary station with two receiveantennas to determine a single preferred precoding vector by derivingthe receive combining vector which maximises the SINR for each transmitbeamforming vector in the codebook. This single preferred precodingvector could typically be the MMSE (minimum mean square estimation)receive combining vector. The UE can report the transmit beamformingvector which maximises the maximum SINR.

For a single stream to one secondary station, this approach can beexpressed as follows:

1. The received signal is given by y=Hgx+n where

-   -   y is the received signal, a N×1 vector    -   x is the transmitted signal, a 1×1 vector    -   g is the precoding vector, M×1    -   H is the channel matrix, N×M    -   n is the noise at each receive antenna, a N×1 vector. For        convenience H can be normalised so that the noise variances are        equal.    -   M is the number of transmit antennas at the eNB    -   N is the number of receive antennas at the UE

2. For each possible g in a codebook of size C, compute the receiveantenna weight vector w (1×N) such that wy={circumflex over (x)}minimises the error E[x−{circumflex over (x)}],i.e.: w=(Hg)^(H)((Hg)(Hg)^(H)+σ² I)⁻¹

3. Report the g which maximises the SINR after computing thecorresponding MMSE solution for w. This is equivalent to reporting g fora single receive antenna, where g is chosen to maximise the receivedSINR for an effective 1×M transmission channel given by wH.

4. The eNB scheduler will select pairs of UEs which report orthogonalg's (or at least g's with low cross-correlation).

In the case of channel vector quantisation (CVQ) based feedback, asimilar approach may result in a single preferred precoding vector forthe feedback. However, this is reliant on an assumption thatzero-forcing beamforming at the primary station transmitter, and relieson an approximation of the resulting SINR.

The main drawback of the above approaches is that they do notnecessarily maximise the sum rate in a cell using MU-MIMO, as a highersum rate might be achieved by choosing a w which enables a differentpairing of UEs but which does not maximise the SINR for each individualUE.

This can be illustrated in FIG. 1 with the beam 151 directed from theprimary station 100 to the secondary station 101. Even if this beam 151is the one maximising the SINR of the secondary station 101, it causeshuge interference on the secondary station 102. This secondary station102 will not be able to have a communication with a high SINR because ofbeam 151 which is directed straight at it.

Moreover, in some cases it is not feasible for the secondary station tocompute a single weight vector w which optimises the SINR, and thereforeit is not feasible to feed back a single preferred transmit precodingvector. Such cases include:

i) the case of a large feedback codebook, such that the number ofdifferent optimisations and SINR calculations becomes prohibitive;

ii) cases where the secondary station does not know the transmitprecoding vector e.g.

a. transmit beamforming at the primary station where the phase referenceis given by precoded reference signals instead of the CRS and anindicator of the actually-used precoding vector; in this case there iseffectively an infinite number of transmit precoding vectors available,for each of which the secondary station would have to derive the optimalweight vector w;

b. channel vector quantisation based feedback, when an assumption ofzero-forcing transmit beamforming may not necessarily be valid.

One aspect of the invention is based on the fact that for the casesidentified above a large or even infinite number of w's are possible.This means that by varying w, it may be possible for the base station toselect pairs of UEs which maximise the sum rate while not necessarilymaximising the rate for any individual UE.

An exemplary variant of a first embodiment of the invention is depictedin FIG. 2, where the primary station 100 is able to direct the beam 151so that the secondary station 102 is not disturbed by it. Even if thebeam 151 does not provide the highest possible SINR value for thesecondary station 101, the sum rate achievable for all the secondarystations can be better since the secondary station 102 is not interferedwith by the beam 151 dedicated to another secondary station, namely 101.

To achieve this, it is proposed, in accordance with a first embodimentof the invention, that the secondary station feeds back to the primarystation a set of preferred precoding vectors, the number of precodingvectors being greater than the preferred rank of transmission. Theprimary station may determine first the preferred rank of thetransmission and configure the secondary station beforehand. Then, thispermits that the secondary station is aware of the number of requiredprecoding vectors that need to be fed back to the primary station. Italso permits to limit the computation requirement at the secondarystation which can be more limited than the primary station in terms ofcomputation power.

It is however possible to let the secondary station decide on thepreferred rank of transmission depending on the state of the channel sothat it permits an optimal use of the channel. In such a case, thesecondary station signals to the primary station the preferred rank oftransmission.

According to a variant of the first embodiment, when applied in the caseof two receive antennas at the secondary station or UE in an LTEnetwork, each UE feeds back two precoding vectors g's, g₁ and g₂, evenwhen rank-1 transmission is preferred. Each precoding vector g can becomputed as above, by selecting two preferably-orthogonal receivevectors w₁ and w₂ which are known or have a relationship which is known,possibly a priori, to both the primary station and the secondarystation.

According to an advantageous embodiment the first receive vector w₁ iscomputed to maximise the rate for a codebook-based feedback approach asdescribed above. A corresponding CQI value, computed using this value ofw, is also fed back, which gives sufficient information for the casewhen no other secondary stations end up being scheduled for transmissionat the same time. The second vector w₂ can then be chosen as anorthogonal vector of w₁ (which gives sufficient information for optimalscheduling of another secondary station), and a second CQI value iscomputed for this value of w and is fed back as well. The secondarystation also feeds back the corresponding g values, g₁ and g₂.

For two receive antennas at the secondary station, a suitable embodimentcould use w vectors w₁=[1 1] and w₂=[1 −1] for example, or [0 1] and [10] corresponding to receive antenna selection.

It is to be noted that this exemplary embodiment of the invention can beextended to a secondary station with N reception antennas, in which casew is a vector of dimension 1×N. In such a case, the secondary stationcould transmit preferred precoding vector feedback corresponding to upto N w vectors. For example, if N=4, the secondary station could feedback 4 preferred precoding vectors, corresponding to w₁, w₂, w₃ and w₄,all of which could for example be orthogonal to each other.

In accordance with a variant of the above example, the secondary stationcould send a reduced amount of feedback corresponding to less than Nw's. In such a case (e.g. for 2 w's), the choice of which particular w'scould take into account the correlation between the receive antennas inorder to maximise the information fed back to the primary station.

For example, if w₁ is selected to maximise the rate, then possiblemultipliers for generating w₂, w₃, and w₄ could be [1 1 −1 −1], [1 −1 1−1], and [1 −1 −1 1]. Using w₂ is likely to be preferable to w₃ or w₄(i.e. it would give the eNodeB more information) assuming that theantennas are indexed in order of separation (and hence correlation).

As a further aspect of the invention, therefore, the secondary stationselects the second w according to the correlation between the antennas(as the primary station does not need to know the relationship betweenthe antenna index and the physical antenna at the secondary station).

In another embodiment, the secondary station selects and feeds back then w's which have the highest SINRs where n<N.

As a further example, if w₁ is selected as [1 1 1 1], then possiblevalues for w₂, and w₄ could be [1 1 −1 −1], [1 −1 1 −1], and [1 −1 −11].

In an embodiment where N=2, the primary station scheduler is then freeto select any g_(A) for user A as a linear combination of g₁ and g₂which orthogonalizes g_(A) and a similarly-derived g_(B) for user B.This can be extended to N>2, where the secondary station reports two (ormore) values of g, and the eNB applies precoding which is a linearcombination of the reported values.

If the secondary station reports N values of g corresponding to N valuesof w, this provides the eNB with some information on the full channelmatrix. However, this has some advantages over known methods, since itis not necessary to specify the ordering of the receive antennas, andthe computational complexity is likely to be lower for equivalentaccuracy of channel representation (i.e. N searches of a codebook sizeC, compared with one search of a codebook size CN).

In a variant of the invention, the primary station is a mobile terminallike a User Equipment, and the primary station is a base station like aneNodeB.

The invention may be applicable to mobile telecommunication systems likeUMTS LTE and UMTS LTE-Advanced, but also in some variants to anycommunication system having allocation of resources to be donedynamically or at least semi persistently.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

The inclusion of reference signs in parentheses in the claims isintended to aid understanding and is not intended to be limiting.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the art of radiocommunication.

The invention claimed is:
 1. A secondary station comprising: a pluralityof antennas, a transceiver configured to communicate in a network with aprimary station in accordance with a preferred rank of transmission, thetransceiver being communicatively coupled to the plurality of antennas,a controller communicatively coupled to the transceiver and configuredto: measure channel state information (CSI) of a downlink channel, andderive a first set of preferred precoding vectors based on the measuredchannel state information (CSI), wherein a number of preferred precodingvectors in the first set of derived preferred precoding vectors isgreater than the preferred rank of transmission, and wherein thepreferred rank of transmission is signaled by the secondary station tothe primary station.
 2. The secondary station of claim 1, wherein atransmission rank indicates the number of parallel layers of spatiallyseparable data streams being simultaneously used for an associated datatransmission between the primary station and the secondary station. 3.The secondary station of claim 1, wherein the preferred rank oftransmission is a desired number of parallel layers of spatiallyseparable data streams being pre-selected for simultaneous use for theassociated data transmission between the primary and the secondarystations.
 4. The secondary station of claim 1, wherein the transceiveris further configured to transmit to the primary station, at least oneindex corresponding to at least one derived preferred precoding vectorfrom among the first set of derived preferred precoding vectors.
 5. Thesecondary station of claim 1, wherein the transceiver is configured totransmit to the primary station, the first set of derived preferredprecoding vectors.
 6. The secondary station of claim 1, wherein the atleast one index is based on one of: a predetermined codebook of transmitbeamforming vectors, or direct channel vector quantization (CVQ).
 7. Thesecondary station of claim 1, wherein the measurement of the channelstate information is based on pre-coded common reference signals (CRS).8. The secondary station of claim 1, wherein the preferred rank oftransmission is configured by the primary station.
 9. The secondarystation of claim 1, wherein the preferred rank of transmission ispredetermined.
 10. The secondary station of claim 1, wherein thesecondary station derives each precoding vector of the derived first setof precoding vectors according to a different corresponding receivecombining vector.
 11. The secondary station of claim 10, wherein thecorresponding receive combining vectors are orthogonal to each other.12. The secondary station of claim 1, wherein the number of derivedfirst precoding vectors is less than the number of receive antennas ofthe secondary station.
 13. The secondary station of claim 1, wherein thesecondary station derives the first set of precoding vectors dependingon at least one or more of: the correlation between the receive antennasof the secondary station, or a corresponding SINR for each precodingvector.
 14. A primary station comprising: a plurality of antennas, acomplex gain amplifier configured to perform beamforming using theplurality of antennas, a transceiver configured to communicate in anetwork with at least a first secondary station, and further configuredto receive from the at least first secondary station a first set ofindices indicative of a corresponding derived set of preferred precodingvectors, wherein the derived set of indices are derived at the at leastfirst secondary station, and wherein a number of preferred precodingvectors is greater than a preferred rank of transmission from theprimary station to the first secondary station, and a controllerconfigured to: generate at least a first transmission precoding vectorbased on a first combination of the first precoding vectors of the firstset, wherein the first set of precoding vectors is derived at the atleast first secondary station, to establish an achievable transmissionrate, generate at least a second transmission precoding vector based ona second combination of the first precoding vectors of a second derivedset of precoding vectors derived at the at least first secondarystation, wherein the second set of derived precoding vectors isindicated in a signaling from the at least first secondary station as asecond set of indices, and wherein the first transmission precodingvector and the second transmission precoding vector are generated at theprimary station so that the sum transmission rate comprising a sum ofthe first secondary station transmission rate and a second secondarystation transmission rate is maximized.
 15. The primary station of claim14, wherein the at least second transmission precoding vector is alinear combination of precoding vectors of the first set of derivedprecoding vectors and the second set of derived precoding vectors. 16.The primary station of claim 15, wherein the first transmissionprecoding vector and the second transmission precoding vector areorthogonal.
 17. The primary station of claim 14, wherein the number ofgenerated precoding vectors is less than the number of receive antennasof the at least first secondary station.
 18. The primary station ofclaim 14, wherein the at least first transmission precoding vector is alinear combination of preferred precoding vectors from among the firstset of preferred precoding vectors generated at the at least firstsecondary station.
 19. The primary station of claim 14, wherein thetransceiver is further configured to receive the at least firstprecoding vector and a corresponding receive combining vector from theat least first secondary station.
 20. The primary station of claim 14,wherein the preferred rank of transmission is configured by the primarystation.
 21. The primary station of claim 14, wherein the preferred rankof transmission is predetermined.
 22. A primary station method ofoperating a primary station for communicating in a network with at leasta first secondary station, the method comprising performing the acts of:in the primary station: receiving via a transceiver through a pluralityof antennas controlled for beamforming by a complex gain amplifier, fromthe at least first secondary station, a first set of indices indicativeof a derived set of preferred precoding vectors, wherein the derived setof indices are derived at the at least first secondary station, andwherein a number of preferred precoding vectors is greater than apreferred rank of transmission from the primary station to the firstsecondary station, and generating at least a first transmissionprecoding vector based on a first combination of the first precodingvectors of the first set, wherein the first set of precoding vectors isderived at the at least first secondary station, to establish anachievable transmission rate, generating at least a second transmissionprecoding vector based on a second combination of the first precodingvectors of a second derived set of precoding vectors derived at the atleast first secondary station, wherein the second set of derivedprecoding vectors is indicated in a signaling from the at least firstsecondary station as a second set of indices, and wherein the firsttransmission precoding vector and the second transmission precodingvector are generated at the primary station so that the sum transmissionrate comprising a sum of the first secondary station transmission rateand a second secondary station transmission rate is maximized.
 23. Asecondary station method of operating a secondary station forcommunicating in a network with a primary station, the method comprisingthe acts of: in the secondary station: measuring via a processor,channel state information (CSI) of a downlink channel, generating viathe processor, a first set of preferred precoding vectors based on themeasured channel state information (CSI), wherein a number of preferredprecoding vectors in the first set of preferred precoding vectors isgreater than a preferred rank of transmission, and transmitting via atransceiver and a plurality of antennas, to a primary station, a set ofindices indicative of the generated first set of preferred precodingvectors, and wherein the preferred rank of transmission is signaled bythe first secondary station to the primary station.
 24. A tangiblecomputer-readable storage medium, that is not a transitive propagatingsignal or wave, modified with control information including instructionsfor operating a primary station to perform a method of communicatingover a network with at least a first secondary station, the methodcomprising, in the primary station, performing the acts of: in theprimary station: receiving via a transceiver through a plurality ofantennas controlled for beamforming by a complex gain amplifier from theat least first secondary station a first set of indices indicative of acorresponding derived set of preferred precoding vectors, wherein thederived set of indices are derived at the at least first secondarystation, and wherein a number of preferred precoding vectors is greaterthan preferred rank of transmission from the primary station to thefirst secondary station, and generating at least a first transmissionprecoding vector based on a linear combination of the derived set ofpreferred precoding vectors to establish an achievable transmission rategenerate at least a second transmission precoding vector based on asecond combination of the first precoding vectors of a second derivedset of precoding vectors derived at the at least first secondarystation, wherein the second set of derived precoding vectors isindicated in a signaling from the at least first secondary station as asecond set of indices, and wherein the first transmission precodingvector and the second transmission precoding vector are generated at theprimary station so that the sum transmission rate comprising a sum ofthe first secondary station transmission rate and a second secondarystation transmission rate is maximized.
 25. A tangible computer-readablestorage medium, that is not a transitive propagating signal or wave,modified with control information including instructions for operating aprimary station to perform a method of communicating over a network withat least a primary station, the method comprising, in the secondarystation, performing the acts of: in the secondary station: measuring viaa processor, channel state information of a downlink channel, derivingvia the processor, a first set of preferred precoding vectors based onthe measured channel state information, wherein a number of preferredprecoding vectors in the first set of preferred precoding vectors isgreater than a preferred rank of transmission, and transmitting to aprimary station, a set of indices indicative of the derived first set ofpreferred precoding vectors, and transmitting to the primary station, apreferred rank of transmission.